Complexes comprising Group VIII noble metals and tertiary organophosphorus compounds are useful for various homogeneous catalytic reactions such as hydrogenation of olefins, carbonyl compounds, aromatic compounds, and the like, and hydroformylation and hydrocarboxylation of olefins. In particular, rhodium-triarylphosphine complexes are commercially advantageously used as catalysts for the hydroformylation of olefins.
These Group VIII noble metal complexes have advantages because of the excellent chemical stability thereof in that a catalyst liquid can be separated from a reaction product by distillation, returned to a reaction zone, and reused, and in that the reaction can be performed continuously while separating the reaction product by distilling away from the reaction zone by gas stripping and allowing the catalyst liquid to remain in the reaction zone. In these reactions, however, various high boiling by-products are formed and the deactivation of catalyst occurs. In carrying out these reactions continuously, therefore, it is necessary to extract a portion of the catalyst liquid continuously or intermittently from the reaction zone since such high boiling by-products and deactivated catalyst accumulate therein.
The thus-extracted catalyst liquid contains expensive Group VIII nobel metals and, therefore, it is very important to efficiently recover such noble metals from an economical viewpoint and furthermore, from a viewpoint of prevention of environmental pollution. In recovering Group VIII noble metals from the extracted catalyst liquid, it is desirable to recover them in the form of a complex which is active for the above-described reactions.
Heretofore known methods of separating and recovering Group VIII noble metal complexes from extracted catalyst liquids include an extraction with a strong acid as described in Japanese Pat. Publication No. 43219/71, and a decomposition with a peroxide compound as described in U.S. Pat. No. 3,547,964 and Japanese Laid-Open Patent Application (OPI) No. 63388/76.
The extraction with a strong acid is a method of recovering a rhodium complex in which a strong acid (for example, 60% by weight or more sulfuric acid) is added to a spent catalyst liquid to extract the rhodium complex therewith, the resulting acid solution containing the rhodium complex, which is separated by phase-separation, is diluted with water to precipitate the rhodium complex, and the precipitate is extracted with a solvent to recover the rhodium complex from the acid solution.
The decomposition method using a peroxide compound comprises treating a spent catalyst liquid with an aqueous solution of an acid, such as nitric acid, and a peroxide compound, isolating the aqueous phase containing a rhodium salt, and after the decomposition with heating of excessive peroxide compound, treating the aqueous phase with carbon monoxide under pressure in the presence of an organic solvent and a complex-forming substance, such as triphenylphosphine, to thereby obtain the rhodium complex in the organic solvent phase.
In either of the above-described methods, the problem of corrosion of apparatus material arises since they use an acid. Furthermore, since the recovered rhodium complex contains sulfate ions (SO.sub.4.sup.2-) and chloride ions (Cl.sup.-), and the sulfur (S) and chlorine (Cl) poison the rhodium complex catalyst, it is necessary to remove them by neutralizing with an alkali.
Japanese Laid-Open patent application (OPI) No. 26218/79 discloses a method in which a still residue of a hydroformylation reaction solution containing therein a rhodium-triarylphosphite complex catalyst is subjected to a pretreatment along with a compatible organic solvent, water, oxygen gas, and a base, heated at 0.degree. to 85.degree. C. until the triarylphosphite is oxidized into a phosphate, and then heated at 115.degree. to 175.degree. C. to thereby precipitate zero valent rhodium, and additionally, a method in which the thus-obtained rhodium precipitate is converted into rhodium oxide (Rh.sub.2 O.sub.3) by application of a procedure of several steps.
This method, however, has disadvantages in that it is very difficult to convert the precipitate of zero valent rhodium, i.e., rhodium in the form of a metal, into a rhodium complex having catalytic activity, and in that the rhodium oxide obtained by application of the above complicated procedure requires a treatment with carbon monoxide and triarylphosphite at high temperature and pressure for the formation of a complex thereof.
As a result of extensive investigations to develop a method of recovering Group VIII noble metal complexes in an organic compound-containing solution efficiently by a simplified method and, furthermore, as a complex which is active for the above-described reactions, it has been found that when the foregoing solution is brought into contact with an oxidizing agent in the presence of a tertiary organophosphorous compound, an organic polar solvent, water and a basic substance, solid complexes of Group VIII noble metals are precipitated.